Project Summary The tumor suppressor p53 transcriptionally regulates hundreds of genes highly associated with its various cellular functions, such as cell cycle arrest, senescence and apoptosis, in response to distinct stresses. However, the kinetics of transcription of p53 targets vary dramatically regardless of their similar p53 RE sequences in most of these target genes. Often, p53 rapidly induces genes important for controlling the cell cycle, but shows delayed action on the expression of apoptotic genes. Although the biological reason for these differential actions of p53 on gene expression seems better understood, i.e., p53 might need to first rescue less genomically damaged cells through a growth arrest mechanism prior to killing them via apoptosis, the mechanisms underlying the delayed transcription of apoptotic genes by p53 remain largely unknown. In an attempt to illustrate the molecular mechanisms, we recently identified a transcriptional elongation factor TFIIS.h, whose gene is TCEA3, as a potential regulator for the expression of some p53 responsive apoptotic genes. TFIIS.h acts as a tumor suppressor in cancer cells, as its overexpression inhibits, while its knockdown promotes, growth of ovarian cancer cells. Also, our preliminary study showed that TFIIS.h is required for the transcription elongation of the bax gene, an apoptotic target of p53, but not of p21, a cell cycle target. In light of literature and our preliminary data, we hypothesize that TFIIS.h as a novel p53 target may selectively activate some of p53 responsive apoptotic genes, but not cell cycle-regulated genes, at the level of transcriptional elongation, which might account for the biochemical and molecular mechanism for why the expression of apoptotic genes, such as Bax, is often delayed in response to p53 activation. We will test this hypothesis by addressing two specific aims. Aim 1. To identify p53 responsive TFIIS.h target genes important for apoptosis. 1.1) To validate TFIIS.h as an authentic p53 target gene: We will employ biochemical and molecular biological approaches and mouse models (p53+/+ and p53-/-) to address this. 1.2) To identify p53 responsive TFIIS.h target genes important for apoptosis. We will do so by conducting ChIP-seq and RNA-seq analyses. Aim 2. To determine the mechanisms underlying selective regulation of transcriptional elongation of p53 responsive genes by TFIIS.h and its tumorigenic role. 2.1) To elucidate mechanisms underlying the transcriptional elongation of p53 targets by TFIIS.h. 2.2) To determine p53-dependent cellular functions of TFIIS.h, such as apoptosis, cell cycle, and senescence, by using TFIIS.h knockout cells generated by CRISPR-Cas systems. 2.3) To determine the biological effect of TFIIS.h on tumor growth. We will do so by using xenograft models. Completing these studies would not only provide new insight into the precise regulation of the kinetics of transcription of p53 target genes associated with different cellular functions, but also offer proof-of-concept evidence for selective induction of apoptosis by TFIIS.h upon p53 activation, very instrumental to the identification of novel targets for developing new therapy for chemo-resistant cancers.